In rat ventricular myocytes, Ca2+ flux balance across stimulation frequencies is maintained by increased efflux due to elevated diastolic [Ca2+]i (contributing 69.3 +/- 6% at 8 Hz) and decreased influx.
In rat ventricular myocytes, cellular Ca2+ flux balance during varying stimulation frequencies is maintained by increased efflux from elevated diastolic Ca2+ and decreased influx via L-type Ca2+ channels.
The force-frequency response is an important physiological mechanism regulating cardiac output changes and is accompanied in vivo by beta-adrenergic stimulation. We sought to determine the role of sarcoplasmic reticulum (SR) Ca2+ content and L-type current (ICa-L) in the frequency response of the systolic Ca2+ transient alone and during beta-adrenergic stimulation. Experiments (on single rat ventricular myocytes) were designed to be as physiological as possible. Under current clamp stimulation SR Ca2+ content increased in line with stimulation frequency (1-8 Hz) but the systolic Ca2+ transient was maximal at 6 Hz. Under voltage clamp, increasing frequency decreased both systolic Ca2+ transient and ICa-L. Normalizing peak ICa-L by altering the test potential decreased the Ca2+ transient amplitude less than an equivalent reduction achieved through changes in frequency. This suggests that, in addition to SR Ca2+ content and ICa-L, another factor, possibly refractoriness of Ca2+ release from the SR contributes. Under current clamp, beta-adrenergic stimulation (isoprenaline, 30 nm) increased both the Ca2+ transient and the SR Ca2+ content and removed the dependence of both on frequency. In voltage clamp experiments, beta-adrenergic stimulation still increased SR Ca2+ content yet there was an inverse relation between frequency and Ca2+ transient amplitude and ICa-L. Diastolic Ca2+i increased with stimulation frequency and this contributed substantially (69.3 +/- 6% at 8 Hz) to the total Ca2+ efflux from the cell. We conclude that Ca2+ flux balance is maintained by the combination of increased efflux due to elevated diastolic Ca2+i and a decrease of influx on IC-L) on each pulse.
Dibb et al. (Fri,) conducted a other in Normal physiology (force-frequency response). Stimulation frequency and beta-adrenergic stimulation (isoprenaline) was evaluated on Systolic Ca2+ transient, SR Ca2+ content, L-type Ca2+ current, and diastolic [Ca2+]i. In rat ventricular myocytes, Ca2+ flux balance across stimulation frequencies is maintained by increased efflux due to elevated diastolic [Ca2+]i (contributing 69.3 +/- 6% at 8 Hz) and decreased influx.